1. Technical Field of the Invention
The present invention relates, in general, to determining the position of a mobile station within a mobile cellular telecommunications network, and specifically to a system and method for more accurately determining mobile station position along border regions within the mobile cellular telecommunications network.
2. Background and Objects of the Invention
Mobile radio position determination is an emerging field that has gained a great deal of interest lately, and it is now desirable to include a position determination capability in future cellular mobile radio systems. The Time Difference of Arrival (TDOA) position determination method, which is known from military applications, has been used successfully for determining the position of mobile radio terminals. A typical TDOA position determination system can be either terminal based, whereby Time Of Arrival (TOA) measurements are made on the "downlink" in the mobile station (MS), or network based, whereby the network performs the TOA measurements on the "uplink" in the radio base transceiver stations (BTSs). These TOA measurements are then used to calculate TDOA parameters and estimate the position of the mobile station (MS).
A network-based method for determining the position of cellular mobile stations is disclosed in commonly-assigned Swedish Patent Application No. 9303561-3 to R. Bodin. In order to determine the position of a mobile station, a handover procedure is initiated between a serving base transceiver station and the new base transceiver station. The mobile station transmits access request signals to a new base transceiver station. The base transceiver station measures the time delay for the access request signal to travel between the mobile station and the base transceiver station. This procedure is repeated between the mobile station and one or more additional base transceiver stations. A service node in the cellular network calculates the position of the mobile station by utilizing information about the known positions of the base transceiver stations and the measured access time delays.
This network-based method of determining the position of cellular mobile stations relies on so-called asynchronous handovers, where the target base transceiver stations measure the access delays to the mobile station. Each access delay is used as a measure of the distance between the mobile station and the respective base transceiver station. In specific situations, i.e., MS positioning, a number of BSs may concurrently tune to the same transmitting MS in order to make time delay of arrival measurements between the MS and the BSs. The time delay of the signal transmitted by the mobile station during a handover procedure may be measured by the BSs for use by a processing unit to determine the position of the mobile station.
Although the above-described system illustrates considerable progress in the cellular position determination field, there are still a number of deficiencies to be improved upon. There are circumstances in existing systems having TOA-based mobile station positioning capabilities in which the determination of a mobile station position may not be highly accurate. Specifically, the location of a mobile station may be most accurately determined by measuring time delays from locations which surround the mobile station to be located and performing a triangulation calculation based upon the measured time delays. Existing systems may potentially measure time delays from locations which fail to surround the mobile station to be located, especially when the mobile station is located near the border between two areas served by separate mobile switching centers (MSCs) or other entities within the network. Consequently, location determinations in such a scenario will not be highly accurate. There exists a need for a system for more accurately determining the position of a mobile station within a mobile cellular telecommunications network.